1. Field of the Invention
The present invention relates to semiconductor devices and methods of manufacturing the semiconductor devices, and more particularly, to semiconductor devices capable of controlling lifetime of carriers and methods of manufacturing the semiconductor devices.
2. Description of the Related Art
In recent years, with the demand for energy saving, the range of applications of power modules used in power conversion apparatuses or the like has expanded. With respect to power devices such as insulated-gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), or freewheeling diodes (FWD) used in the power modules, in order to improve trade-off between speed and power loss in switching characteristics, it is important to control lifetime of carriers.
As a technique of controlling the lifetime of the carriers, there is a method of intentionally introducing crystal defects in a drift region of a device by electron-beam irradiation. As the defects introduced by the electron-beam irradiation, composite defects are generated due to combination of carbon and oxygen originally contained in a wafer. However, the depth of a defect level is different according to the type of the composite defects, the degree of influence on lifetime of the carriers is also different.
Due to difference of wafer manufacturing methods for every wafer makers, impurity concentrations of carbon, oxygen, and the like contained in the wafers are different among wafer makers. For this reason, the composition ratios of the composite defects generated after the electron-beam irradiation are different according to wafers, and thus, there is a problem that the lifetime of the carriers is different. In addition, even in the case of wafers purchased from the same wafer maker, impurity concentrations are different for each ingot of semiconductor crystals, and impurity concentrations are different according to different portions in even the same ingot, so that the same problem occurs.
In order to remove variations in device characteristics due to such difference in impurity concentration of carbon and the like, a method capable of allowing variations in impurity concentration of carbon originally contained in the wafer to be neglected by introducing a large amount of carbon into the wafer is proposed (refer to JP 2006-352101 A).
However, in a method described in JP 2006-352101 A, since device characteristics are changed by adding a process of introducing a large amount of carbon into a wafer, the method cannot be applied to devices that have already been in mass production, and costs are greatly increased.
In view of the above problems, an object of the present invention is to provide a semiconductor device capable of equalizing composition ratios of various types of composite defects having different levels among the processed wafers after electron-beam irradiation and easily adjusting variations in device characteristics even in a case where impurity concentrations of carbon or oxygen contained in a base material (semiconductor substrate) as a starting material are different and a method of manufacturing the semiconductor device.